The present invention relates generally to communication systems, and in particular, to a method for calculating capacity variation and effective channel rate per sector in connection with the soft handoff rate in a CDMA network.
Mobile communication networks and personal communication systems employing CDMA technology do not limit accommodation capacity to a fixed value, but rather, accommodate user flexibly in accordance with the constantly changing network environment. CDMA technology makes it possible for a mobile to maintain a simultaneous connection (i.e., occupy a radio link) with two or three base stations as it transitions between them. This process is referred to as soft handoff and represents an important characteristic of CDMA technology. However, one effect of soft handoffs is that the accommodation capacity of a CDMA network and base station varies in accordance with the soft handoff rate. A drawback of conventional cellular mobile communication networks is that call-drops frequently occur when a mobile station experiences a serious fading within a handoff area. To overcome this, CDMA technology utilizes soft handoffs to improve the percentage of successful handoffs in CDMA networks. Soft handoffs provide an additional advantage in eliminating audible xe2x80x9cimpulsexe2x80x9d noises that are often heard in mobile stations during handoff procedures.
However, CDMA networks have other problems, in that network capacity is decreased by the soft handoff method employed. This is because a mobile station performing a soft handoff occupies two or three radio links at the same time and accordingly utilizes the capacity of two or three stations during this procedure. This means that the capacity of a CDMA network may vary according to the ratio of soft handoff areas in a network to total service area in a network or by the rate at which channels within base stations perform soft handoffs.
Conventional CDMA network design methods have used the terms xe2x80x9cratio of soft handoff area to total service areaxe2x80x9d and xe2x80x9csoft handoff channel ratexe2x80x9d interchangeably. For example, in conventional CDMA network design methods, the soft handoff channel rate is assumed to be 40% which has resulted in the ratio of the soft handoff area to the total service area also assumed to be 40%. Adopting these values decreases the call success rate over the whole network. The number of users who can communicate simultaneously is also reduced as the accommodation capacity of whole network is significantly reduced as compared with a conventional radio network design. Therefore, CDMA networks often require additional equipment to achieve the service quality that the network was designed for, or service quality can only be maintained with a smaller number of users than contemplated in the original system design.
Other examples of conventional CDMA network design approaches include U.S. Pat. No. 5,710,758 (xe2x80x9cthe ""758 patentxe2x80x9d). In the ""758 patent, a method for simulating and analyzing the operation of a CDMA wireless network is described. In particular, an electronic representation of a wireless telecommunications system is configured for a given market area and the operation of that wireless telecommunications system simulated. An analysis of the reverse and forward links are performed to determine which locations within a demand and service vector will be able to establish reverse and forward links, respectively, with base stations placed at desired locations throughout the service area.
In contrast to the present invention, the ""758 patent is directed to the design of a CDMA network and, as such, does not address accommodation capacity and/or the handoff rate.
The present invention provides a method for establishing and/or optimizing the handoff rate and accommodation capacity of a CDMA network by calculating capacity variation and effective channel rate per sector as a function of the soft handoff channel rate.
The soft handoff channel rate is used as a basis for determining the handoff rate and the accommodation capacity because it can be easily converted to a ratio of soft handoff area to the total service area. The ratio then readily confirms whether the target design value of accommodation capacity can be satisfied.
In accordance with a first aspect of the present invention, there is provided a method for determining the number of effective channels, and an effective channel rate in a code division multiple access network, comprising the steps of: calculating the ratio of the soft handoff area to the total service area, HAT; calculating the soft handoff channel rate, HCT as a function HAT; and determining the number of the effective channels, NEFFECTIVE, and the effective channel rate, RCH-EFFECTIVE, as a function of HCT and HAT.
According to the method of the present invention, the ratio of the soft handoff area to the total service, HAT is related to xcex94R and D such that xcex94R=Dxc2x7(1xe2x88x92{square root over (1xe2x88x92HAT)}), where 0xe2x89xa6HATxe2x89xa61 where R is a cell radius, D is a radius of coverage that the corresponding sector can service with the best signal quality, C is a radius of coverage that only the corresponding sector can service and xcex94R is radius-difference between R and D or between C and D satisfying 0 xcex94R D.
In accordance with the present invention, HAT, the ratio of the soft handoff area to the total service, is preferably calculated as the sum of HA3 and HA2 where HA3 is the ratio of the 3-way soft handoff area in the total service area, and HA2 is the ratio of the 2-way soft handoff area in the total service area.
HA3 is preferably derived as a function of HAT as HA3=2xc2x7(1xe2x88x92{square root over (1xe2x88x92HAT)})2 and HA2 is preferably derived as a function of HAT as HA2=HATxe2x88x922xc2x7(1xe2x88x92{square root over (1xe2x88x92HAT)})2.
The soft handoff channel rate is preferably calculated as a function of HAT       H    CT    =                    4        ·                  (                      1            -                                          1                -                                  H                  AT                                                              )                                      (                      2            -                                          1                -                                  H                  AT                                                              )                2              .  
HCT is preferably calculated as the sum of HC3 and HC2, HCT=HC3+HC2, where HC3 is a 3-way handoff channel ratio for the total service area, and the HC2 is a 2-way handoff channel ratio for the total service area.
The 3-way handoff channel ratio, HC3 is related to xcex94R and D such that HC3=6xc2x7(AR)2/(D+xcex94R)2, where R is a cell radius, D is a radius of coverage that the corresponding sector can service with the best signal quality, C is a radius of coverage that only the corresponding sector can service and xcex94R is the radius-difference between R and D or between C and D satisfying 0 xcex94R D.
HC2 is preferably derived as a function of       H    AT    :                              (                      1            -                                          1                -                                  H                  AT                                                              )                ·                  (                                    6              ·                                                1                  -                                      H                    AT                                                                        -            2                    )                                      (                      2            -                                          1                -                                  H                  AT                                                              )                2              .  
The number of effective channels, NEFFECTIVE is preferably derived as NEFFECTIVE=NMAXxc2x7((1xe2x88x92HCT)+HC2/2+HC3/3), where
NMAX is the maximum number of channels at a system loading environment defined in a practical system design,
HCT is the handoff channel rate,
HC2 is the 2-way handoff channel ratio for the total service area and the HC3 is the 3-way handoff channel ratio for the total service area.
In accordance with another aspect of the present invention, the number of effective channels, NEFFECTIVE is derived as a function of             H      AT        :          N      EFFECTIVE        =            N      MAX        ·                  1                              (                          2              -                                                1                  -                                      H                    AT                                                                        )                    2                    .      
In accordance with another aspect of the present invention, the effective channel rate, RCH-EFFECTIVE is derived as RCH-EFFECTIVE=(1xe2x88x92HCT)+HC2/2+HC3/3, where HCT is handoff channel rate, HC2 is 2-way handoff channel ratio for the total service area and, HC3 is 3-way handoff channel ratio for the total service area.
In accordance with another aspect of the present invention, the effective channel rate, RCH-EFFECTIVE is derived as a function of HAT, the ratio of the soft handoff area to the total service area:       R          CH      -      EFFECTIVE        =            1                        (                      2            -                                          1                -                                  H                  AT                                                              )                2              .  
A preferable embodiment of the method of the present invention for designing a CDMA communication system includes:
(a) calculating the number of effective channels, NEFFECTIVE as NEFFECTIVE=NMAXxc2x7((1xe2x88x92HCT)+HC2/2+HC3/3), where:
NMAX is the maximum number of channels that can reasonably be accommodated in a realizable CDMA communication system
HCT is handoff channel rate,
HC2 is the 2-way handoff channel rate for the total service area and
HC3 is the 3-way handoff channel rate among total service area;
(b) calculating the effective channel rate, RCH-EFFECTIVE=(1xe2x88x92HCT)+HC2/2+HC3/3.
A preferable embodiment of the present invention of the method for optimizing the CDMA network in the CDMA digital mobile communication system includes:
(a) calculating the number of effective channels, NEFFECTIVE as a function of HAT such that             N      EFFECTIVE        =                  N        MAX            ·              1                              (                          2              -                                                1                  -                                      H                    AT                                                                        )                    2                      ,
xe2x80x83were the NMAX is the maximum number of channels at a system loading environment defined in a practical system design, and HATis a ratio of the soft handoff area to the total service; and
(b) calculating the effective channel rate, RCH-EFFECTIVE as a function of HAT as       R          CH      -      EFFECTIVE        =            1                        (                      2            -                                          1                -                                  H                  AT                                                              )                2              .  
In accordance with another embodiment of the present invention, there is provided a method for designing a CDMA network, comprising the steps of determining the area that the CDMA network is intended to service; arbitrarily selecting one or more base stations for use in the CDMA network and a transmission power for each selected base station; calculating radius values R, C and D from the transmission power of each base station and predetermined network parameters; calculating the number of base stations required for the network based on said determined area and radius values; calculating values for NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT for the CDMA network; if the values for NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT fall within pre-established target value bounds, ending the design process; otherwise, adjusting at least one of: the placement of the base stations, the transmission power, and the network parameters, and re-calculating values for NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT.
Preferably, said step of adjusting comprises the steps of: choosing a value for at least one of NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT that falls within the pre-established target value bounds; calculating values for NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT if values were not pre-determined; if said values for NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT do not fall within the target bounds, returning to said choosing step; otherwise calculating radius values for R, C and D to satisfy the values for NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT; and adjusting at least one of: the placement of the base stations, the transmission power, and the network parameters, and re-calculating values for NEFFECTIVE, RCH-EFFECTIVE, HAT and HCT.